Layered monodiphosphate Li9V3(P2O7)3(PO4)2: A novel cathode material for lithium-ion batteries

Single phase Li9V3(P2O7)3(PO4)2 is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: P3¯c1) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10−8 S cm−1 is higher than that of LiFePO4 and as the same order of Li3V2(PO3)4. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0-4.6 V and 2.0-4.8 V, respectively. In the range of 2.0-4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g−1 of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li9V3(P2O7)3(PO4)2. In the range of 2.0-4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.